SO LONG, LOSERS Rare is the television star who heeds Kenny Rogers' "know when to fold 'em" rule. Jerry Seinfeld, who signed off this time last year to much fanfare, was the exception. Those shuffling off the screen this year are being pushed by precipitous ratings drops. Was it a change in the zeitgeist? The fact that they had enough episodes for syndication (all had more than 100)? Or did each, like Rhoda, make a fatal flub?